Multi-band portable subscriber units (SUs) are becoming available that can communicate in a wireless local area network (WLAN) as well as in a wide area network (WAN), such as a cellular network. Such SUs “register” with the WLAN and communicate through the WLAN when within the coverage area of the WLAN. When outside the coverage area of the WLAN, such SUs are “de-registered” from the WLAN, and communicate through the WAN.
When the SU does not have a call in progress, the registration and de-registration processes are relatively straightforward. In one prior-art embodiment, the SU is expected to re-register periodically with the WLAN. In the absence of a timely re-registration, the WLAN de-registers the SU and will forward all future calls for the SU to the WAN, until the SU again registers with the WLAN. A problem arises, however, when the SU has a call in progress or a new call is received for the SU just as the SU is about to leave the coverage area of the WLAN. Such calls are likely to be dropped.
Thus, what is needed is a method and apparatus for facilitating a handoff of the SU from the WLAN to the WAN. The handoff preferably will provide a seamless, standards-compliant mobility between the two administrative domains when the SU is leaving the coverage area of the WLAN and moving into the WAN.